This invention relates to an apparatus for braking systems, incorporating a pressure-control device for anti-skid control, which comprises an inlet port adapted to communicate with the pressure-control device, an outlet port adapted to connect with at least one wheel brake, and a throttle providing for permanent communication between the inlet port and the outlet port. Such an apparatus is already known from German Pat. No. 909,657.
In braking systems incorporating pressure-control devices for antiskid control, the general problem is that the pressure-control device should provide for braking-pressure decrease and increase as rapidly as possible, ensuring on the other hand that the wheel is not immediately brought to a lock again. In the known apparatus, this is achieved by a piston slidably sealed in a cylinder and having a throttle extending therethrough, with a spring bearing on the piston in the direction of the pressure-control device. By the provision of a check valve which is inserted before the cylinder it is ensured that fluid is allowed to flow through the cylinder only in the direction towards the wheel brake. By means of a check valve arranged in parallel with the cylinder, fluid is only allowed to return from the wheel brake to the pressure-control device. In this arrangement, the known apparatus acts in such a manner that the pressure exerted by a braking pressure source is passed to the cylinder through the preceding check valve, thereby displacing the piston in opposition to the spring, which causes braking pressure to build up in the wheel brake. In rapidly succeeding antiskid control cycles, however, the piston bottoms the cylinder and renewed build up of braking pressure is only possible in the degree permitted by the throttle.
If the braking pressure is completely exhausted during an antiskid control cycle, a large amount of pressure fluid will be required initially until pressure builds up in the wheel brake because, for example, the clearance of the wheel brake has to be overcome first and because more pressure fluid is employed as a result of elastic deformations in the lower pressure range. While it is true that, at the commencement of a braking action and during prolonged intervals between the antiskid control cycles as well as prolonged phases of pressure decrease, the known apparatus permits quick supply of pressure fluid to the wheel brake by displacement of the piston, a certain amount of time will nevertheless lapse in the case of rapidly succeeding antiskid control cycles until braking pressure will be build up again if it had dropped to as low as zero value. This is of sufficient disadvantage in itself. But considering further that in the case of braking-pressure decrease to a partial value the subsequent pressure increase will commence immediately, it will be readily seen that this apparatus cannot be satisfactory. It is a further disadvantage therein that the proportion of rapid pressure increase depends on how far the piston happened to have been pushed back by the spring during the respective antiskid control cycle.